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July 17, 1956 J. R. TAVlS 2,755,443

GALVANOMETER Filed Dec. 15, 1952 INVENTOR. JOHN R. TA V/S A T TORNE Y rw I iinite states GALVANOMETER Application December 15, 1952, Serial No. 326,08ii

4 Claims. (Cl. 324-97) This invention relates to improvements in galvanometers of the type adapted to respond to alternating or oscillating voltages, and more particularly to the type of laboratory and portable galvanometers finding widespread use in oscillographs.

A typical galvanometer of this type comprises a lightweight fine wire coil held in suspension between a pair of stretched wires or ribbons respectively anchored at opposite ends to an enclosing galvanometer case. The suspension means defines the axis of rotary motion of the coil and supports a small mirror which is generally symmetrically arranged about this axis and for defiection responsive to rotation of the coil. A window in the galvanorneter case aligned with the mirror enables incidence and reflection of a light beam at the mirror for sensing coil displacement. A pair of magnet pole pieces are mounted through opposite walls of the case to define a narrow gap interiorly of the case and within which the coil is located. The pole pieces extend outwardly of the case for suitable engagement with magnet means as of an oscillograph.

The coil suspension means in the form of conductive wire or ribbon provide electrical leads to the coil, the upper suspension means conventionally being anchored to one terminal post sealed through an end of the galvano-meter case and providing one external contact. The lower suspension means is egnerally resiliently anchored, as by a captive spring, the tension of which may be adjusted by means or" a screw passing through the bottom of the case and into a sliding wafer to which one end of the spring is anchored. In present practice a fine wire lead is attached to the spring or spring anchoring means and is carried upwardly in the casing and through a side wall thereof into an exteriorly mounted terminal housing wherein it is mechanically held in contact with a second terminalrpost. The terminal housing is normally mounted at the upper end of the galvanometer case in proximity to the first external contact, described above, to facilitate connection in an oscillograph.

The principle of operation of such a galvanometer together with its manner of'use is thoroughly familiar in the art.

it is the constant aim in the design of galvanometers of this character to increase sensitivity without sacrifice of durability and ruggedness. l have discovered means for increasing both sensitivity and ruggedness of galvanometers of the type described in the form of a new system for providing external electrical contact with the lower suspension means. The invention is characterized by replacement of purely mechanical contact, as heretofore employed, at both ends of the electrical lead from the lower suspension means with bonded contacts and the structure necessary to enable such replacement. At one and the same time, and as a consequence of this arrangement, there results an improvement in sensitivity due to avoidance of high resistance inherent in mechanical contacts and an increase in ruggedness of the instrument by reduction of the possibility of mechanical failure.

2,755,443 Patented July 17, 1956 The invention contemplates in a galvanometer having a case, a wire coil suspended between magnet polesin the case by upper and lower suspension means, and a terminal housing mounted to and exteriorly of the case and including a terminal post, the combination comprising an intermediate semi-rigid electrical lead connected to and extending from the terminal post through a Wall of the case and inwardly thereof, and a fine wire lead connected at one end to the lower suspension means and at the opposite end to the intermediate lead. In a preferred embodiment the Wire lead is bonded both at the point of contact with the lower suspension means and at a point of contact with the intermediate lead, the point of contact being within the case. Bonded contact is considered to include any form of welding, soldering, etc. as distinguished from mechanically induced contact.

The invention also contemplates in combination with a galvanometer of the type described, a case having an opening in a wall thereof, a terminal housing mounted on the exterior of the case and covering the opening, a terminal post supported by the housing, an intermediate lead connected at one end to the terminal post and passing through the housing and the opening in the wall of the case, and means connecting an electrical lea-d extending from the lower suspension means to the intermediate lead interiorly of the case. Again preferred practice consists of bonding the intermediate lead to the terminal post and the wire lead to the lower suspension means and to.

the intermediate lead.

The invention will be more clearly understood with reference to the accompanying drawing in which:

Fig. l is an elevation of a galvanometer of the type to which the invention is directed, with a portion of the front wall of the case cut away;

Fig. 2 is a plan view of the instrument of Fig. 1;

Fig. 3 is a partial vertical section taken on the line 33 of Fig. 2, showing in detail a form of terminal housing in accordance wtih the invention; and

Fig. 4 is an enlarged partial elevation of the lower portion of the galvanometer of Fig. 1 with the front wall of the case removed.

The galvanometer shown in elevation and in plan view respectively in Figs. 1 and 2 comprises a case 10 in the form of a square or rectangular sectioned box, the length of which is many times its cross-sectional dimension. The case comprises a rear wall 11 and'side Walls 12 and 13. A cover 14, of substantially the same shape as the back member 11, is slidably engaged in grooves 12A, 13A respectively of the side members 12 and 13. The cover is held on the case by a screw 15 and may be removed for access to the interior of the case by removal of the screw 15.

A pair of pole pieces 16, 17 are fastened through the sides 12 and 13 respectively of the case defining a relatively narrow gap 18 within the case symmetrically arranged about the longitudinal aXis thereof. Pole pieces 16 and 17 extend outwardly from the side walls 12 and 13 of the case and are adapted for engagement in a suitable magnet block (not shown).

An elongated coil 20 consisting of a large number of fine wire windings and shaped to occupy a very small cross-sectional area is suspended in the gap 18 between the pole pieces. The coil is suspended .in the gap between opposing upper and lower suspension wires 22, 23 respectively, the suspension wires being connected to the coil termini to provide electrical leads to the coil. A small rectangular mirror at is attached to the upper suspension wire in alignment with a window (not shown) in the side wall 13 of the case whereby a beam of light may be directed on the mirror through one or the other of rotatably mounted lenses 28, 29 and reflected back through the respective lens for sensing deflection of the coil 20.

The upper suspension wire is anchored to a terminal post 32 sealed through the upper end of the casing and providing one external contact. The detailed construction of the terminal post 32 and the manner of mounting the same through the end of case forms no part of the present invention.

The lower suspension wire 23, as most clearly shown in Fig. 4 which is an enlarged view of the lower portion of the galvanometer of Fig. 1, is bonded to a hook 34 which in turn engages one end of a helical spring 35, the spring being aflixed at its opposite end to a slidable wafer 36. A block 38 is mounted at the bottom end of the case and supports an adjusting screw 39 threadably engaging the wafer 36. By means of the screw 39, tension on spring 35 and suspension wire 23 may be adjusted by displacing the wafer 36 longitudinally in the case. The block 38 also provides a receptacle for a cap screw (see Fig. 1).

In accordance with the invention a fine wire lead 42 is connected at one end to hook 34 and is hence independent of mechanical contact between the hook and the spring 35. Preferably the lead is bonded to the hook so that mechanical contact is entirely eliminated. Lead 42 is carried upwardly in the case for connection to a second external contact in the manner most clearly shown in Fig. 3. Fig. 3 is a partial sectional elevation taken on the line 33 of Fig. 2.

A terminal housing 44 in accordance with the invention comprises an insulating block 45 having a longitudinal bore 46 extending from end to end thereof and including a reduced diameter section 46A lying between upper and lower larger diameter sections 46B, 46C respectively. A pair of spaced transverse bores 43, 49 intersect the longitudinal bore 46 and provide means for fixing the housing to the wall 12 of the galvanometer case, as by means of screws 50, 51 respectively.

The case wall 12 is provided with a small port 52 about which the terminal housing 44 is mounted and the housing block 45 has an internal passageway 54 communicating with the section 46B of the longitudinal bore and with a side wall of the block at a point aligned with the port 52 in the case wall. A terminal post 56 is mounted in section 463 of the longitudinal bore and is held therein by a screw 57 inserted within section 46C of the bore and extending through the reduced diameter central section 46A of the bore to engage in the lower end of the terminal 56. A washer 58 is normally interposed between the head of screw 57 and the annular shoulder formed by the junction of sections 46A, 46C of the longitudinal bore.

The terminal post 56 has a cavity 56A formed on a surface thereof adjacent its inner end conveniently by grinding a fiat portion. A semi-rigid wire 60 is bonded at one end in this cavity 56A and extends from the terminal post 56 through passageway 54 and port 52 into case 10. The fine wire lead 42 carried upwardly in the case from the lower end of lower suspension wire 23 (see Fig. 4) is bonded to the inwardly projecting end of the wire 60, as by soldering, and hence need not be carried through the wall of case 10. By way of comparison of the sizes of the lead wire 42 and the intermediate lead 60, the former may have a diameter of approximately .002 inch and the latter a diameter of approximately .010 inch.

The result of the arrangement as illustrated is that the lower suspension wire 23 and hence the lower terminal of the coil 20 is connected to the exterior terminal 56 through a continuous bonded conductive system devoid of any solely mechanical contact. The intermediate lead extending into the housing from the terminal post 56 is considerably more rugged than the interior lead 42 as a consequence of a diameter of about five times that of the interior lead, with a consequent advantage of mechanical ruggedness at the junction. Moreover, the use of such an intermediate lead permits a bonded connection as described, which is impossible in present galvanometer construction.

I claim:

1. In a galvanometer having a wire coil suspended between magnet poles by opposed suspension means, a galvanometer case enclosing the coil and the suspension means, and a terminal housing mounted on the side of the case, the combination which comprises a terminal post slidable longitudinally in the direction of the suspension into a recess in the terminal housing, a first flexible electrical lead metallically bonded to and projecting from the inside end of the terminal post through an aperture in the terminal housing and a second aperture in the wall of the case into the interior of the case, the axes of they apertures being at an angle to that of the terminal post, a second electrical lead disposed inside the case and fastened at one end to one of the suspension means and metallically bonded at the other end to the first electrical lead, and an access closure on the case adjacent the metallic bond between the first and second leads, the first electrical lead being slidable through the apertures into the case as the terminal post slides in the recess.

2. Apparatus according to claim 1 in which the first electrical lead is of substantially greater cross section than the second electrical lead.

3. Apparatus according to claim 1 provided with means for locking the terminal post in the terminal housing.

4. Apparatus according to claim 1 provided with a screw passing from the outside of the housing into the inner end of the terminal post and threaded into the post for locking it in place in the housing.

References Cited in the file of this patent UNITED STATES PATENTS 1,877,243 Ising Sept. 13, 1932 2,503,298 Richardson Apr. 11, 1950 2,550,720 Richardson May 1, 1951 FOREIGN PATENTS 166,653 Great Britain July 11, 1921 

